Mr Brown, who taunted the Tory leader for riding his bike to work while a car followed with his bags, said: "When are the Conservative Party going to be honest when they say they support green taxes and then they run away from every one of them?

"You are sounding more and more like a used car salesman today."

Mr Cameron replied: "It's not my backbenchers who are telling me to get on my bike."

We haven't got a problem with green taxes but they have got to be fair and the retrospective bit of this tax is not fair

Ronnie CampbellLabour MP

He said the road tax rise was "not a green tax, it's a stealth tax". "What on earth is green about taxing someone who bought a Ford Mondeo five years ago?"

Mr Brown replied that the shake up of vehicle excise duty would save 1.3m tonnes of CO2 and increase the number of clean cars.

But speaking to the BBC later, Labour MP Ronnie Campbell said up to 80 Labour MPs were unhappy about the plans and urged Mr Brown to look at the plans again.

He fears they could provoke a backlash similar to that against the abolition of the 10p tax rate.

"This affects working people with second-hand cars. When they have to come next year and renew their road tax, they are going to get a pretty big shock when they have got another £100, £150 to £200 to pay," he said.

"We haven't got a problem with green taxes but they have got to be fair and the retrospective bit of this tax is not fair."

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